Speech in noise-filled environments is a frequently known problem of hearing-impaired persons, which here require a signal-to-noise ratio of one to 10 dB in order to achieve the same speech intelligibility as persons with normal hearing. Moreover the natural directional effect of the outer ear is lost in the case of coverage using behind-the-ear hearing devices. The rehabilitation using hearing devices is thus not only to include the individual compensation of the hearing loss by means of amplification and dynamic compression but also the reduction of noises, in order to bring about a significant improvement in the speech intelligibility in noisy situations. Modern digital hearing devices exhibit noise suppression methods, which satisfy the hearing device-specific requirements in terms of efficiency, sound quality and artifact freedom.
Directional microphones in this way rank among noise suppression methods which were established several years ago and clearly lead to improvements in the speech intelligibility in auditory situations, in which the wanted signal and the interference signals come from different directions. In modern hearing devices, the directional effect is generated by differential processing of two or more adjacent microphones with omnidirectional characteristics. In “Störgeräuschreduktion bei Hörsystemen der Gegenwart”, [Noise reduction in present day hearing systems] Chapter “Directional microphone systems”, 5th DGA Annual Convention 2002, differential 1st and 2nd order systems as well as systems with adaptive directional characteristics are described.
FIG. 1 shows a simplified block diagram of a 1st order directional microphone system comprising two microphones 1, 2 at a distance of approximately 10 to 15 mm. An external delay of T2 herewith occurs between the first and second microphone for acoustic signals coming from the front V, said delay corresponding to the distance of microphones 1, 2 in respect of each other for instance. The signal R2 of the second microphone 2 is delayed by the time T1 in the delay unit 3, is inverted in the inverter 4 and is added to the signal R1 of the first microphone 1 in the first adder 5. The total produces the directional microphone signal RA, which can be fed to a receiver by way of a signal processing for instance. The direction-dependent sensitivity essentially develops from a subtraction of the second microphone signal R2 delayed by time T2 from the first signal R1. Acoustic signals from the front V are thus not damped after suitable equalization, whereas acoustic signals from the rear S are deleted for instance. The design and efficiency of directional microphone systems for hearing devices are described in the patent application DE 103 31 956 B3 for instance.